1. Field of the Invention
The invention concerns electrical insulators and more particularly those of which the dielectric materials are glass or porcelain.
2. Description of the Prior Art
It is known that atmospheric pollution may lead to the formation of conductive deposits on the surface of insulators.
As the electrical resistance at the level of the surface layer of the insulator is not uniform, there is observed in moist conditions the presence of dry areas in series with moist areas.
There may then be produced in these dry areas voltage gradients which are very much higher than in the moist areas, likely to reach the threshold for breakdown in air.
Moreover, when the extent of the dry areas reaches a certain proportion of the length of the insulator, there then occurs complete flashover of the latter resulting in a short-circuit across the network which takes it out of service.
In order to overcome these disadvantages it has already been proposed, in U.S. Pat. No. 3 795 499 for porcelain insulators and in British Pat. No. 1 240 854 for synthetic plastic insulators, to coat the surface of the dielectric material with a semiconductor layer with a resistivity which does not vary with the current, such as a semiconductor enamel, for example, so as to juxtapose to the polluted layer of irregular resistivity an underlying layer of constant resistivity in order to control the distribution of potential along the insulator.
This solution is not fully satisfactory, however.
If the current passing through the semiconductor layer is not significantly higher than that passing through the polluted layer, the semiconductor layer has virtually no effect, since it is the polluted layer which determines the distribution of potential, in an irregular manner.
On the other hand, if the current in the semiconductor layer is significantly higher than that in the polluted layer, the phenomena resulting from the juxtaposition of the dry areas and the moist areas cannot occur, but the energy losses are then too high for this solution to be economically acceptable. Moreover, this solution is not durably reliable.
It is therefore necessary to adopt a compromise solution, which is satisfactory only for cases of slight pollution.
Also, with a semiconductor coating of defined resistivity it is only possible, in cases of serious pollution, to attenuate the defects explained hereinabove and not to eliminate them.
The present invention makes it possible to remedy these disadvantages.